全文获取类型
收费全文 | 4556篇 |
免费 | 67篇 |
国内免费 | 34篇 |
专业分类
化学 | 2788篇 |
晶体学 | 38篇 |
力学 | 77篇 |
数学 | 467篇 |
物理学 | 1287篇 |
出版年
2019年 | 34篇 |
2018年 | 21篇 |
2017年 | 24篇 |
2016年 | 67篇 |
2015年 | 43篇 |
2014年 | 57篇 |
2013年 | 182篇 |
2012年 | 179篇 |
2011年 | 227篇 |
2010年 | 95篇 |
2009年 | 87篇 |
2008年 | 207篇 |
2007年 | 223篇 |
2006年 | 201篇 |
2005年 | 201篇 |
2004年 | 190篇 |
2003年 | 169篇 |
2002年 | 151篇 |
2001年 | 139篇 |
2000年 | 127篇 |
1999年 | 67篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 91篇 |
1995年 | 72篇 |
1994年 | 95篇 |
1993年 | 105篇 |
1992年 | 99篇 |
1991年 | 58篇 |
1990年 | 57篇 |
1989年 | 80篇 |
1988年 | 56篇 |
1987年 | 51篇 |
1986年 | 59篇 |
1985年 | 73篇 |
1984年 | 56篇 |
1983年 | 34篇 |
1982年 | 57篇 |
1981年 | 53篇 |
1980年 | 57篇 |
1979年 | 49篇 |
1978年 | 55篇 |
1977年 | 66篇 |
1976年 | 44篇 |
1975年 | 43篇 |
1974年 | 50篇 |
1973年 | 49篇 |
1972年 | 31篇 |
1971年 | 26篇 |
1970年 | 24篇 |
排序方式: 共有4657条查询结果,搜索用时 15 毫秒
941.
Mary E. Neubert Sandra S. Keast Julie M. Kim Kyle J. Miller Rachel M. Murray Aaron G. Norton Raj A. Shenoy Margaret E. Walsh Rolfe G. Petschek 《Liquid crystals》2004,31(2):175-184
A limited selection of ring modified diphenyldiacetylenes of the type where A=, Y=CnH2n+1, CF3, F, COMe, NH2, and NMe2, and A=, trans and cis with Y=F and trans with Y=C3H7, were synthesized. Mesomorphic properties were determined by hot stage polarizing microscopy and DSC. These properties were generally poorer than those found in the parent benzene compounds. This was also true of some pyrimidine analogues reported earlier. Birefringence values also decreased as expected. 相似文献
942.
Richard G. Zepp P. F. Schlotzhauer M. S. Simmons G. C. Miller G. L. Baughman N. L. Wolfe 《Fresenius' Journal of Analytical Chemistry》1984,319(2):119-125
Summary Light-induced transformations of organic chemicals have been studied to provide kinetic results concerning the influence of sunlight on pollutant behavior in lakes and rivers. A review of these studies shows that the humus component of the dissolved organic matter in natural water affects aquatic photoreactions in two important ways: by attenuating sunlight and by sensitizing photoreactions. Studies of humus-sensitized photoreactions of furans, sulfides, dienes, nitroaromatic compounds, and other chemicals are discussed. Research concerning effects of aquatic particulates on pollutant photoreaction rates are reviewed. Suspended sediments mainly affect photolysis rates through light attenuation and scattering. Algae, diatoms, and bacteria accelerate light-induced transformations of anilines and phosphorothioate insecticides, suggesting that photobiological processes may play a significant role in the breakdown of certain pollutants in aquatic environments. Evidence is presented that nitrate, iron species, and peroxides may be responsible for sunlight-initiated free radical oxidations of pollutants in some natural waters.
Dynamik der Photoreaktionen von Verunreinigungen in der Hydrosphäre
Zusammenfassung Um kinetische Daten über den Einfluß von Sonnenlicht auf das Verhalten von Verunreinigungen in Seen und Flüssen zu erhalten, wurden licht-induzierte Umwandlungsreaktionen von organischen Chemikalien untersucht. Es ergab sich, daß die Humuskomponente der in natürlichen Wässern gelösten organischen Substanzen auf zwei Arten die aquatischen Photoreaktionen beeinflußt. durch Schwächung des Sonnenlichtes und durch Sensibilisierung der Photoreaktionen. Humus-sensibilisierte Photoreaktionen von Furanen, Sulfiden, Dienen, nitroaromatischen Verbindungen u.a. werden diskutiert. Ein Überblick wird gegeben über die Wirkung aquatischer Partikel auf die Geschwindigkeit der Photoreaktion von Verunreinigungen. Suspendierte Sedimente wirken hauptsächlich durch Lichtabschwächung und Streuung auf die Photolysegeschwindigkeit. Algen, Diatomeen und Bakterien beschleunigen lichtinduzierte Umwandlungen von Anilinen und Phosphorthioat-Insecticiden, was darauf hindeutet, daß photobiologische Prozesse eine bedeutende Rolle beim Abbau gewisser Verunreinigungen in aquatischer Umgebung spielen könnten. Es wird gezeigt, daß Nitrat, Eisenverbindungen und Peroxide für die Sonnenlicht-induzierte Oxidation freier Radikale von Verunreinigungen in natürlichen Wässern verantwortlich sein können.相似文献
943.
944.
The nonconforming Wilson's brick classically is restricted to regular hexahedral meshes. Lesaint and Zlamal[6] relaxed this constraint for the two-dimensional analonue of this element In this paper we extend their results to three dimensions and prove that and where u is the exact solution, u_h is the approximate solution and is the usual norm for the Sobolev space H~1(?). 相似文献
945.
This paper describes a course offered to teachers of American Indian students, which focused on the development of culturally relevant activities as part of the science and mathematics curricula. In response to the concern that American Indian students do not find meaning in the curriculum, these activities were embedded in a holistic approach to the curriculum, and the informal science and mathematics of the culture were linked with the traditional school science and mathematics. Informal results suggest that the development of these connections will help American Indian students make sense of what they are learning, both in the context of the culture and in the context of school science and mathematics. 相似文献
946.
947.
Yang Xi Beckwith Andrew W. Miller John H. Wood Lowell T. 《Central European Journal of Physics》2004,2(4):636-644
The charges in live cells interact with or produce electric fields, which results in enormous dielectric responses, flexoelectricity,
and related phenomena. Here we report on a contraction of Schizosaccharomyces pombe (fission yeast) cells induced by magnetic fields, as observed using a phase-sensitive projection imaging technique. Unlike electric fields, magnetic fields only act
on moving charges. The observed behavior is therefore quite remarkable, and may result from a contractile Lorentz force acting
on diamagnetic screening currents. This would indicate extremely high intracellular charge mobilities. Besides, we observed
a large electro-optic response from fission yeast cells. 相似文献
948.
D L Miller 《The Journal of the Acoustical Society of America》1998,104(4):2498-2505
The ultrasonic activation of free microbubbles, encapsulated microbubbles, and gas-filled micropores was explored using available linear theory. Encapsulated microbubbles, used in contrast agents for diagnostic ultrasound, have relatively high resonance frequencies and damping. At 2 MHz the resonance radii are 1.75 microns for free microbubbles, 4.0 microns for encapsulated microbubbles, and 1.84 microns for gas-filled micropores. Higher-pressure amplitudes are needed to elicit equivalent subharmonic, fundamental, or second-harmonic responses from the encapsulated microbubbles, and this behavior increases for higher frequencies. If an encapsulated microbubble becomes destabilized during exposure,the resulting liberated microbubble would be about twice the linear resonance size, which would be likely to produce subharmonic signals. Scattered signals used for medical imaging purposes may be indicative of bioeffects potential: The second harmonic signal is proportional to local shear stress for a microbubble on a boundary, and a strong subharmonic signal may imply destabilization and nucleation of free-microbubble cavitation activity. The potential for bioeffects from contrast agent gas bodies decreases rapidly with increasing frequency. This information should be valuable for understanding of the etiology of bioeffects related to contrast agents and for developing exposure indices and risk management strategies for their use in diagnostic ultrasound. 相似文献
949.
950.